Direct digital synthesizers (DDSs), or numerically controlled oscillators (NCOs), are a functional requirement of many digital communications systems, including modems and software defined radios. Frequency synthesis is commonly realized using application specific parts (ASSPs) or software executing on a digital signal processing (DSP) processor. With the increasing capabilities of PLDs, for example, field programmable gate arrays (FPGAs), DSP functions are increasingly being implemented on PLDs. PLDs have the advantages of speed that approaches the speed of ASIC implementations along with the programmability associated with DSP processors.
Quadrature oscillators (a type of DDS) are used for constructing digital down converters, demodulators, and various types of modulation schemes, such as phase shift keying, frequency shift keying, and minimum shift keying. With careful design, a DDS can also be used in systems that require frequency hopping and chirp waveform synthesis. One method for digitally generating a complex or real valued sinusoid employs a look-up table. The look-up table stores samples of a sinusoid, and a digital integrator is used to generate a suitable phase argument that is mapped by the look-up table to the desired output waveform.
The fidelity of a signal formed by recalling samples of a sinusoid from a look-up table is affected by both the phase and amplitude quantization of the process. The depth and width of the look-up table affect the signal's phase angle resolution and the signal's amplitude resolution respectively. The depth of the look-up table is the number of sample points stored therein, and the width of the look-up table is the number of bits used to represent each sample. These resolution limits are equivalent to time base jitter and to amplitude quantization of the signal. The resolution limits add spectral modulation lines and a white broad-band noise floor to the signal's spectrum.
It would therefore be desirable to implement a circuit arrangement that controls these undesired spectral terms, and which minimizes usage of logic resources.